1. Field of the Invention
The present invention relates to an air-fuel ratio control method for an internal combustion engine.
2. Description of the Background Information
In a known air-fuel ratio control device as disclosed in Japanese Patent Publication No. 55-3533, for example, an exhaust gas component concentration such as an oxygen concentration in an exhaust gas is detected by an exhaust gas component concentration sensor, and an air quantity or a fuel quantity in a fuel mixture to be supplied to an internal combustion engine mounted in a vehicle is regulated according to a detection value by the exhaust gas component concentration sensor to thereby feedback control an air-fuel ratio of the supplied fuel mixture, for the purposes of purification of the exhaust gas and improvement in fuel consumption, etc.
An internal combustion engine mounted on a vehicle is sometimes restarted under a high engine temperature condition caused by prolonged cruising of the vehicle. In this case, there is a possibility that the fuel in a float chamber and a fuel passage of a carburetor is vaporized owing to the high temperature during the stoppage of the engine, and fuel bubbles operate to discharge a large amount of the fuel from a nozzle by a pumping action, which is generally called a percolation phenomenon. As a result, the air-fuel ratio of the supplied fuel mixture is enriched upon the engine-start under the high engine temperature, causing an increase in emission of unburnt components such as CO (carbon monoxide) and HC (hydrocarbon), etc.
Furthermore, when the intake air temperature is high at idling after the engine-start at a high engine temperature, the air-fuel becomes over-rich because of a decrease of the air density of the intake air as well as the generation of the fuel vapor from the carburetor. Additionally, as a replenishment efficiency in an engine combustion chamber is decreased, the air-fuel ratio is further enriched. Moreover, an air-fuel ratio correction unitary quantity by proportional-plus-integral control at idling is small, and an air-fuel ratio correction speed by a secondary air or the like is slow. Therefore, there occurs a reduction in engine speed at idling after starting at a high engine temperature as mentioned above. Since the overrichness of the air-fuel ratio continues until the vapor deposited on the inner wall of the intake pipe is completely absorbed, the emission of especially CO and HO is increased during this period of time. On the other hand, the air-fuel ratio of the supplied fuel mixture is enriched because of the percolation phenomenon thereby to cause degradation in engine-start property at restarting of the engine under a high engine temperature condition. Then, there is disclosed in Japanese Patent Laid-Open Publication No. 58-185965 an air-fuel ratio control device to improve the engine-start property at high engine temperature. In this air-fuel ratio control device, a secondary air control valve of an ON-OFF type flow regulating valve is provided in a secondary intake air supplying passage communicating an intake manifold, and the secondary air control valve is maintained open for a predetermined time (1.0-1.5 sec) at engine-restart under the high engine temperature, thus preventing the enrichment of the air-fuel ratio by supplying a secondary intake air.
However, in the air-fuel ratio control device having a linear type flow regulating valve capable of continuously changing the opening degree thereof in the secondary intake air supplying passage, the linear type flow regulating valve can control a flow quantity per unit time greater than the ON-OFF type flow regulating valve. Therefore, when the linear type flow regulating valve is maintained in its full open position at engine-restart under the high engine temperature, the air-fuel ratio of the supplied fuel mixture is rendered overlean, causing degradation in engine-start property.
Further, even when the same quantity of the secondary intake air is supplied at the same engine temperature upon engine restarting under the high engine temperature, the engine-start property is dependent upon the atmospheric temperature. Particularly, it has been found that the higher the atmospheric temperature is, the more the engine-start property is deteriorated.